Fuel cell system and method of operating the fuel cell system

ABSTRACT

A fuel cell system includes a first heating mechanism and a second heating mechanism. The first heating mechanism supplies a reformer with some of an exhaust gas discharged from a fuel cell stack as a heat source for directly heating the reformer. The second heating mechanism supplies the remaining exhaust gas to the heat exchanger and utilizes the heat generated in the heat exchanger as a heat source for indirectly heating the reformer. Temperature sensors are attached to the reformer. An open/close valve is adjusted based on the temperatures detected by the temperature sensors to control the ratio between the amount of heat supplied from the first heating mechanism to the reformer and the amount of heat supplied from the second heating mechanism to the reformer.

TECHNICAL FIELD

The present invention relates to a fuel cell system including a fuelcell stack, a heat exchanger, and a reformer. Further, the presentinvention relates to a method of operating the fuel cell system.

BACKGROUND ART

For example, a solid oxide fuel cell (SOFC) employs an electrolyte ofion-conductive oxide such as stabilized zirconia. The electrolyte isinterposed between an anode and a cathode to form an electrolyteelectrode assembly (unit cell). The electrolyte electrode assembly isinterposed between separators (bipolar plates). In use, predeterminednumbers of the unit cells and the separators are stacked together toform a fuel cell stack.

Normally, as a fuel gas supplied to the fuel cell, a hydrogen gasproduced from a hydrocarbon based raw fuel by a reforming apparatus isused. In the reforming apparatus, after a reforming raw material gas isobtained from the hydrocarbon based raw fuel such as a fossil fuel,e.g., methane or LNG, the reforming raw material gas is subjected tosteam reforming or partial oxidation reforming, autothermal reforming orthe like to produce a reformed gas (fuel gas).

Normally, the city gas used as a raw fuel contains, in addition tomethane (CH₄), hydrocarbon of high carbon (C₂₊) such as ethane (C₂H₆),propane (C₃H₆), and butane (C₄H₁₀). In the case of using the hydrocarbonof high carbon as a fuel of a solid oxide fuel cell, hydrocarbon of C₂₊should be removed by reforming. It is because the carbon may beprecipitated in the fuel pipe or on the anode to degrade the cellperformance undesirably. In this case, the water vapor needs to besupplied excessively.

In this regard, for example, Japanese Laid-Open Patent Publication No.2003-229163 discloses a preliminary reformer. In a steam reformingmethod of the preliminary reformer, reforming catalyst is filled in thepreliminary reformer. In the preliminary reformer, an exhaust air from afuel cell is utilized as a required heating source to remove hydrocarbonof C₂₊ from a fuel.

In the conventional technique, the preliminary reformer is operated inthe temperature range of about 300° C. to 600° C. The S/C (steam/carbon)ratio is adjusted to be in the range of 1.5 to 6.0. That is, since theoperating temperature of the preliminary reformer is high, in order toavoid precipitation (coking) of carbon, the S/C ratio needs to beconsiderably high. Thus, it is necessary to supply water excessively forthe reforming reaction. Therefore, the capacity of the water supplypower source such as a water pump needs to be large. As a result, theload of the fuel cell is large uneconomically.

DISCLOSURE OF INVENTION

A main object of the present invention is to provide a fuel cell systemand a method of operating the fuel cell system in which the reformingcondition is maintained, and the amount of supplied water is reduced asmuch as possible.

The present invention relates to a fuel cell system including a fuelcell stack, a heat exchanger, and a reformer. The fuel cell stack isformed by stacking a plurality of fuel cells. Each of the fuel cellsincludes an electrolyte electrode assembly and a separator stackedtogether. The electrolyte electrode assembly includes an anode, acathode, and an electrolyte interposed between the anode and thecathode. The heat exchanger heats an oxygen-containing gas to besupplied to the fuel cell stack. The reformer reforms a mixed fuel of araw fuel chiefly containing hydrocarbon and water vapor to produce areformed gas. Further, the present invention relates to a method ofoperating the fuel cell system.

Firstly, some of the exhaust gas discharged from the fuel cell stackafter consumption in power generation reaction is supplied to thereformer as a heat source for directly heating the reformer. Theremaining exhaust gas is supplied to the heat exchanger as a heat sourcefor heating the oxygen-containing gas, and the heat generated in theheat exchanger is supplied to the reformer as a heat source forindirectly heating the reformer. The indirect heating herein meansheating by the radiation heat or the convection heat.

Then, the temperature of the reformer is detected, and the flow rate ofthe exhaust gas supplied to the reformer is adjusted based on thedetected temperature of the reformer. Thus, the state of the reformer ismaintained to have predetermined reforming condition values.

Further, preferably, the reformer and the heat exchanger are providednear the fuel cell stack, and the heat exchanger is provided outside thereformer to form an indirect heating space of a second heating mechanismbetween the heat exchanger and the reformer. In the structure, theradiation heat or the convection heat is supplied from the heatexchanger to the reformer through the indirect heating space, and thereformer is heated desirably.

Further, preferably, the reformer has an inlet for allowing the mixedfuel to flow into the reformer through the inlet, and an outlet forallowing the reformed gas after reforming to be supplied to the fuelcell stack through the outlet. Preferably, the inlet is providedadjacent to an exhaust gas outlet of a first heating mechanism. Inparticular, in the structure, the inlet of the reformer which tends tohave the lowest temperature by endothermic reaction of steam reformingis heated preferentially. Thus, the sharp decrease in the temperature isprevented, and the uniform temperature in the reformer is maintained.Accordingly, the reforming reaction occurs stably, and the S/C ratio ismaintained at a certain level.

Further, preferably, the reforming condition value comprises thetemperature of the reformer, and the molar ratio of carbon and the watervapor to the raw fuel. Thus, it is possible to lower the S/C ratio downto the coking limit. It is possible to reduce the amount of suppliedwater, and the load of the fuel cell is reduced effectively.

Further, preferably, the fuel cell system further includes an evaporatorfor evaporating water to produce the mixed fuel. Preferably, a fluidunit including the heat exchanger, the evaporator, and the reformer isprovided on one side of the fuel cell stack, and the fluid unit isprovided symmetrically with respect to the central axis of the fuel cellstack. In the structure, the components of the fluid unit having thehigh temperature are provided locally within the same area. Heatdissipation from the fluid unit is prevented. Thus, improvement in theheat recovery rate is achieved. Further, significant heat stress or heatdistortion is not generated, and improvement in the durability isachieved.

Preferably, the flow direction of the remaining exhaust gas along theheat exchanger is substantially in parallel with the flow direction ofthe mixed fuel along the reformer. In the reformer, the temperature onthe upstream side in the flow direction of the mixed fuel is the lowest.In the heat exchanger, the temperature on the upstream side in the flowdirection of the remaining exhaust gas is the highest. Therefore, thetemperature of the reformer becomes uniform along the flow direction ofthe mixed fuel by the heat from the heat exchanger.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically showing a fuel cell system accordingto a first embodiment of the present invention;

FIG. 2 is a partial cross sectional view showing the fuel cell system;

FIG. 3 is a cross sectional view showing main components of a fluid unitof the fuel cell system;

FIG. 4 is a perspective view schematically showing a fuel cell stack ofthe fuel cell system;

FIG. 5 is an exploded perspective view showing a fuel cell of the fuelcell stack;

FIG. 6 is a partial exploded perspective view showing gas flows in thefuel cell;

FIG. 7 is a perspective view showing main components of an evaporator ofthe fuel cell system;

FIG. 8 is a partial cross sectional view showing a reformer of the fuelcell system;

FIG. 9 is an exploded perspective view showing main components of thereformer;

FIG. 10 is a graph showing the relationship between the temperature andthe position in the reformer, in the case of regulating the temperaturein the longitudinal direction of the reformer; and

FIG. 11 is a cross sectional view showing main components of a fluidunit of a fuel cell system according to a second embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A fuel cell system 10 is used in various applications, includingstationary and mobile applications. For example, the fuel cell system 10is mounted on a vehicle. As shown in FIGS. 1 and 2, the fuel cell system10 includes a fuel cell stack 12, a fluid unit 14 provided on one sideof the fuel cell stack 12, and a casing 16 containing the fuel cellstack 12 and the fluid unit 14.

As shown in FIGS. 2 and 3, the fluid unit 14 includes a heat exchanger18 for heating an oxygen-containing gas before it is supplied to thefuel cell stack 12, an evaporator 20 for evaporating water to produce amixed fuel of raw fuel chiefly containing hydrocarbon (e.g., the citygas) and the water vapor, and a reformer 22 for reforming the mixed fuelto produce a reformed gas.

The reformer 22 is a preliminary reformer for producing a raw fuel gaschiefly containing methane (CH₄) using hydrocarbon of high carbon (C₂₊)such as ethane (C₂H₆), propane (C₃H₆), and butane (C₄H₁₀) in the citygas by steam reforming. The operating temperature of the reformer 22 isin the range of 200° C. to 500° C., more preferably 300° C. to 400° C.,further more preferably at 350° C. The S/C ratio in the reformer 22 ispreferably 1.0.

In the casing 16, a load applying mechanism 24 is provided on the otherside of the fuel cell stack 12 for applying a tightening load in astacking direction of the fuel cells 26 of the fuel cell stack 12indicated by an arrow A (see FIGS. 2 and 4). The fluid unit 14 and theload applying mechanism 24 are provided symmetrically with respect tothe central axis of the fuel cell stack 12.

The fuel cell 26 is a solid oxide fuel cell. As shown in FIGS. 5 and 6,the fuel cell 26 includes electrolyte electrode assemblies 36. Each ofthe electrolyte electrode assemblies 36 includes a cathode 32, an anode34, and an electrolyte (electrolyte plate) 30 interposed between thecathode 32 and the anode 34. For example, the electrolyte 30 is made ofion-conductive oxide such as stabilized zirconia.

The operating temperature of the fuel cell 26 is high, about 700° C. ormore. In the electrolyte electrode assembly 36, hydrogen is produced bydirectly reforming methane in the fuel gas at the anode 34 and thehydrogen is utilized at the anode 34 for power generation reaction.

A plurality of, e.g., eight electrolyte electrode assemblies 36 aresandwiched between a pair of separators 38 to form the fuel cell 26. Theeight electrolyte electrode assemblies 36 are arranged in a circleconcentric with a fuel gas supply passage 40 extending through thecenter of the separators 38. An oxygen-containing gas supply unit 41 isprovided hermetically around the fuel gas supply passage 40.

In FIG. 5, for example, each of the separators 38 comprises a singlemetal plate of, e.g., stainless alloy or a carbon plate. The fuel gassupply passage 40 extends through the center of the separators 38. Theseparator 38 includes a plurality of circular disks 42. Each of thecircular disks 42 has first protrusions 48 on its surface which contactsthe anode 34. The first protrusions 48 form a fuel gas channel 46 forsupplying the fuel gas along an electrode surface of the anode 34.

Each of the circular disks 42 has second protrusions 52 on its surfacewhich contacts the cathode 32. The second protrusions 52 form anoxygen-containing gas channel 50 for supplying the oxygen-containing gasalong an electrode surface of the cathode 32. As shown in FIGS. 5 and 6,each of the circular disks 42 has a fuel gas inlet 54 for supplying thefuel gas to the fuel gas channel 46.

A channel member 56 is fixed to the separator 38 by brazing or laserwelding on a surface facing the cathode 32. The fuel gas supply passage40 extends through the center of the channel member 56. The channelmember 56 forms a fuel gas supply channel 58 connecting the fuel gassupply passage 40 and the fuel gas channel 46. An exhaust gas dischargechannel 59 is formed around the separators 38 for discharging consumedreactant gases as an exhaust gas.

As shown in FIGS. 2 and 4, the fuel cell stack 12 includes a pluralityof the fuel cells 26 stacked together, and end plates 60 a, 60 bprovided at opposite ends in the stacking direction. A hole 61 is formedat the center of the end plate 60 a, and holes 62 and screw holes 64 areformed alternately at predetermined angular intervals along the samevirtual circle around the hole 61. The holes 62 are connected to an airchannel 84 as described later.

As shown in FIG. 2, the casing 16 includes a first case unit 66 acontaining the load applying mechanism 24 and a second case unit 66 bcontaining the fuel cell stack 12. The end plate 60 b and an insulatingmember (not shown) are sandwiched between the first case unit 66 a andthe second case unit 66 b. The insulating member is provided on the sideof the second case unit 66 b. The joint portion between the first caseunit 66 a and the second case unit 66 b is tightened by screws 68 andnuts 70.

The second case unit 66 b is joined to one end of a cylindrical thirdcase unit 72 as part of the fluid unit 14. A head plate 74 is fixed tothe other end of the third case unit 72. An exhaust gas channel 76 isprovided in the third case unit 72. The exhaust gas after consumption inthe power generation discharged from the exhaust gas discharge channel59 of the fuel cell stack 12 flows through the exhaust gas channel 76 inthe fluid unit 14.

As shown in FIGS. 1 and 3, the exhaust gas channel 76 is connected to afirst heating mechanism 77 a for supplying some of the exhaust gas tothe reformer 22 as a heat source for directly heating the reformer 22,and connected to a second heating mechanism 77 b for supplying theremaining exhaust gas to the heat exchanger 18 as a heat source forheating the oxygen-containing gas, and supplying the heat generated inthe heat exchanger 18 to the reformer 22 as a heat source for indirectlyheating the reformer 22.

The first heating mechanism 77 a includes a first channel 78 connectedto the exhaust gas channel 76 and the second heating mechanism 77 bincludes a second channel 80 connected to the exhaust gas channel 76.Further, a third channel 82 is connected to the downstream side of thesecond channel 80 for supplying the exhaust gas to the evaporator 20 asa heat source for evaporating water. The second channel 80 is a mainpassage, and the first channel 78 is branched from the second channel 80through a plurality of holes 81 a formed in a wall 81. The first channel78 is opened to the reformer 22 through a rectification hole (exhaustgas outlet) 83 (see FIG. 3).

The reformer 22 and the evaporator 20 are arranged in the directionindicated by the arrow A1 such that the reformer 22 is positioned on theside of the fuel cell stack 12, and the evaporator 20 is positioned onthe side away from the fuel cell stack 12. The heat exchanger 18 isprovided outside the reformer 22. An indirect heating space 85 as partof the second heating mechanism 77 b is formed between the heatexchanger 18 and the reformer 22.

The distance between the heat exchanger 18 and the reformer 22, and thefuel cell stack 12 should be minimized. The exhaust gas dischargechannel 59 of the fuel cell stack 12 is directly connected to the secondchannel 80 of the second heating mechanism 77 b through the exhaust gaschannel 76. The second channel 80 is provided inside the heat exchanger18. Further, an air channel 84 for the passage of the air is providedinside the heat exchanger 18, near the second channel 80. In thestructure, the exhaust gas and the air heated by the exhaust gas flow ina counterflow manner. The air channel 84 is connected to the air supplypipe 86 at the head plate 74.

The evaporator 20 has an outer cylindrical member 88 and an innercylindrical member 90. The outer cylindrical member 88 and the innercylindrical member 90 are coaxial with each other. A double pipe 92 isprovided spirally between the outer cylindrical member 88 and the innercylindrical member 90. As shown in FIGS. 3 and 7, the double pipe 92includes an outer pipe 94 a and an inner pipe 94 b. The third channel 82is formed between the outer pipe 94 a, and the outer cylindrical member88 and the inner cylindrical member 90.

A raw fuel channel 96 is formed between the outer pipe 94 a and theinner pipe 94 b. A water channel 98 is formed inside the inner pipe 94b. The inner pipe 94 b has a plurality of holes 100 on the downstreamside of the evaporator 20. For example, the diameter of the holes 100 isin the range of 10 μm to 100 μm.

An end of the double pipe 92 on the upstream side extends through thehead plate 74 to the outside. At an end of the double pipe 92 on thedownstream side, the inner pipe 94 b is terminated, and only the outerpipe 94 a extends in the direction indicated by the arrow A2. An end ofa mixed fuel supply pipe 101 is connected to the outer pipe 94 a, andthe other end of the mixed fuel supply pipe 101 is connected to an inlet102 of the reformer 22 (see FIG. 3). The mixed fuel supply pipe 101extends toward the fuel cell stack 12, and is connected to the inlet102. The inlet 102 is provided near the rectification hole 83 connectedto the first channel 78 branched from the exhaust gas channel 76. Theflow direction of the mixed fuel along the reformer 22 is substantiallyparallel to the flow direction of the exhaust gas along the heatexchanger 18, as indicated by the arrow A1.

As shown in FIG. 8, the reformer 22 has a lid 108, and the inlet 102 isformed at the lid 108. The lid 108 is positioned at an end of thereformer 22, and the reformer 22 is formed by connecting first receivermembers 110 and second receiver members 112 alternately. As shown inFIGS. 8 and 9, the first and second receiver members 110, 112 have asubstantially plate shape. A hole 114 is formed at the center of thefirst receiver member 110. A plurality of holes 116 are formed in acircle in the peripheral portion of the second receiver member 112.

A plurality of reforming catalyst pellets 118 are sandwiched between thefirst and second receiver members 110, 112. Each of the catalyst pellets118 has a columnar shape. For example, the catalyst pellet 118 is formedby providing a nickel based catalyst on the base material of ceramicscompound.

A reforming channel 120 is formed in the reformer 22. The reformingchannel 120 extends in the direction indicated by the arrow A1, and hasa serpentine pattern going through the holes 114 of the first receivermembers 110 and the holes 116 of the second receiver members 112. On thedownstream side of the reformer 22 (at the end of the reformer 22 in thedirection indicated by the arrow A1), an outlet 122 is provided, and anend of a reformed gas supply passage 124 is connected to the outlet 122(see FIG. 8). As shown in FIG. 3, the reformed gas supply passage 124extends along the axis of the reformer 22, into the hole 61 of the endplate 60 a, and is connected to the fuel gas supply passage 40.

A main exhaust gas pipe 126 and an exhaust gas pipe 128 are connected tothe head plate 74. The main exhaust gas pipe 126 is connected to thethird channel 82 of the evaporator 20. The exhaust gas pipe 128 isprovided at the center of the evaporator 20 for discharging the exhaustgas flowing around the reformer 22 in the direction indicated by thearrow A1.

A cylindrical cover 129 is provided around the outer cylindrical member88 of the evaporator 20. A heat insulating layer 129 a is formed in aclosed space between the cylindrical cover 129 and the outer cylindricalmember 88. The heat insulating layer 129 a is connected to the secondchannel 80, and some of the exhaust gas is filled in the heat insulatinglayer 129 a.

As shown in FIG. 1, the fuel cell system 10 includes a plurality oftemperature sensors (temperature detection unit) 130 a to 130 d fordetecting the temperature of the reformer 22, such as thermocouples, anopen/close valve (valve mechanism) 132 for regulating the flow rate ofthe exhaust gas supplied to the first heating mechanism 77 a, and acontrol device (valve control mechanism) 134 for regulating the ratiobetween the amount of heat supplied from the first heating mechanism 77a to the reformer 22 and the amount of heat supplied from the secondheating mechanism 77 b to the reformer 22 by adjusting the open/closevalve 132 in order to maintain the state of the preliminary reformer tohave predetermined reforming condition values.

The temperature sensor 130 a is provided near the inlet 102 of thereformer 22. The temperature sensors 130 b to 130 d are successivelydisposed along the flow direction of the mixed fuel gas in the reformer22 in the direction indicated by the arrow A1. The temperature sensor130 d is provided at the nearest position from the outlet 122 of thereformer 22. The open/close valve 132 is provided at the exhaust gaspipe 128. Incidentally, instead of the open/close valve 132, a variablevalve having different valve positions for changing the opening degreeof the valve may be provided at the exhaust gas pipe 128.

As shown in FIG. 2, the load applying mechanism 24 includes a firsttightening unit 140 a for applying a first tightening load to a regionaround (near) the fuel gas supply passage 40 and a second tighteningunit 140 b for applying a second tightening load to the electrolyteelectrode assemblies 36. The second tightening load is smaller than thefirst tightening load.

As shown in FIGS. 2 and 4, the first tightening unit 140 a includesshort first tightening bolts 142 a screwed into screw holes 64 formedalong one diagonal line of the end plate 60 a. The first tighteningbolts 142 a extend in the stacking direction of the fuel cells 26, andengage a first presser plate 144 a. The first presser plate 144 a is anarrow plate, and engages the central position of the separator 38 tocover the fuel gas supply passage 40.

The second tightening unit 140 b includes long second tightening bolts142 b screwed into screw holes 64 formed along the other diagonal lineof the end plate 60 a. Ends of the second tightening bolts 142 b extendthrough a second presser plate 144 b having a curved outer section. Nuts146 are fitted to the ends of the second tightening bolts 142 b. Springs148 and spring seats 149 are provided in respective circular portions ofthe second presser plate 144 b, at positions corresponding to theelectrolyte electrode assemblies 36 on the circular disks 42 of the fuelcell 26. For example, the springs 148 are ceramics springs.

Operation of the fuel cell system 10 will be described below.

As shown in FIGS. 3 and 7, a raw fuel such as the city gas (includingCH₄, C₂H₆, C₃H₈, and C₄H₁₀) is supplied to the raw fuel channel 96 ofthe double pipe 92 of the evaporator 20, and water is supplied to thewater channel 98 of the double pipe 92. Further, an oxygen-containinggas such as the air is supplied to the air supply pipe 86.

In the evaporator 20, the raw fuel moves spirally along the raw fuelchannel 96 in the double pipe 92, the water moves spirally along thewater channel 98, and the exhaust gas as described later flows throughthe third channel 82. Thus, the water moving through the water channel98 is evaporated, and gushes out from a plurality of holes 100 formed onthe downstream side of the inner pipe 94 b to the raw fuel channel 96.

At this time, the water vapor is mixed with the raw fuel flowing throughthe raw fuel channel 96, and the mixed fuel is obtained. The mixed fuelis supplied to the inlet 102 of the reformer 22 through the mixed fuelsupply pipe 101 connected to the outer pipe 94 a. As shown in FIG. 8,the mixed fuel supplied from the inlet 102 into the reformer 22 flowsthrough the hole 114 of the first receiver member 110. The mixed fuel isreformed by the catalyst pellets 118 interposed between the first andsecond receiver members 110, 112. Further, the mixed fuel is supplied tothe next pellets 118 from the holes 116 formed in the peripheral portionof the second receiver member 112.

Thus, the mixed fuel moving along the reforming channel 120 having theserpentine pattern in the reformer 22 is reformed by steam reforming.Thus, hydrocarbon of C₂₊ is eliminated to produce a reformed gas (fuelgas) chiefly containing methane. The reformed gas flows through thereformed gas supply passage 124 connecting to the outlet 122 of thereformer 22. Then, the reformed gas is supplied to the fuel gas supplypassage 40 of the fuel cell stack 12. The reformed gas supply passage124 extending along the axis of the reformer 22 is heated by the exhaustgas branched from the first channel 78 and the heat radiated from theheat exchanger 18. The temperature of the reformed gas supply passage124 is increased to the temperature range (e.g., 600° C.) where powergeneration reaction occurs.

As shown in FIGS. 5 and 6, the fuel gas from the fuel gas supply passage40 flows along the fuel gas supply channel 58. The fuel gas flows fromthe fuel gas inlet 54 of the circular disk 42 into the fuel gas channel46. In each of the electrolyte electrode assemblies 36, the fuel gasinlet 54 is formed at substantially the central position of the anode34. Therefore, the fuel gas is supplied from the fuel gas inlet 54 tothe substantially center of the anode 34. Thus the methane in the fuelgas is reformed to produce a hydrogen gas. The fuel gas chieflycontaining the hydrogen moves along the fuel gas channel 46 toward theouter region of the anode 34.

As shown in FIG. 3, when the air supplied from the air supply pipe 86 tothe heat exchanger 18 moves along the air channel 84 of the heatexchanger 18, heat exchange is carried out between air and the burnedexhaust gas as descried later flowing along the second channel 80. Thus,the air is heated to a predetermined temperature. As shown in FIGS. 5and 6, the air heated in the heat exchanger 18 is supplied to theoxygen-containing gas supply unit 41 of the fuel cell stack 12, andflows into a space between the inner circumferential edge of theelectrolyte electrode assembly 36 and the inner circumferential edge ofthe circular disk 42 in the direction indicated by the arrow B.Therefore, the air flows from the inner circumferential edge to theouter circumferential edge of the cathode 32 along the oxygen-containinggas channel 50.

Thus, in the electrolyte electrode assembly 36, the fuel gas flows alongthe anode 34, and the air flows along the cathode 32 for generatingelectricity by electrochemical reactions at the anode 34 and the cathode32. The exhaust gas is discharged to the outside of each of theelectrolyte electrode assemblies 36, and flows in the stacking directionalong the exhaust gas discharge channel 59. Then, the exhaust gas flowsinto the exhaust gas channel 76.

The exhaust gas flowing through the exhaust gas channel 76 has the hightemperature of about 700° C. As shown in FIG. 3, the exhaust gaspartially flows into the first channel 78 branched through the hole 81a. The exhaust gas is supplied into the inlet 102 of the reformer 22from the rectification hole 83 of the wall 81. After the exhaust gaslocally heats the inlet 102 of the reformer 22, the exhaust gas flowsinside the evaporator 20, and is discharged to the outside from theexhaust gas pipe 128.

Further, the remaining exhaust gas supplied to the second channel 80 ofthe exhaust gas channel 76 flows through the heat exchanger 18. Heatexchange between the exhaust gas and the air is performed. The air isheated to a predetermined temperature, and the temperature of theexhaust gas is decreased. Some of the exhaust gas is filled in the heatinsulating layer 129 a, and the remaining exhaust gas flows into thethird channel 82 connected to the second channel 80. The third channel82 is formed between the outer cylindrical member 88 and the innercylindrical member 90 of the double pipe 92 of the evaporator 20. Theexhaust gas evaporates the water flowing through the water channel 98 ofthe double pipe 92. Therefore, it is possible to reliably produce themixed fuel of the raw fuel and the water vapor in the raw fuel channel96. After the exhaust gas flows through the evaporator 20, the exhaustgas is discharged to the outside through the main exhaust gas pipe 126.

In the first embodiment, as shown in FIG. 1, the first heating mechanism77 a and the second heating mechanism 77 b are provided. The firstheating mechanism 77 a directly supplies some of the exhaust gasdischarged into the exhaust gas channel 76, to the reformer 22. Thesecond heating mechanism 77 b supplies the remaining exhaust gas to theheat exchanger 18, and supplies the heat generated in the heat exchanger18 through the space 85 to the reformer 22 as the heat source forindirectly heating the reformer 22.

In the structure, the reformer 22 is directly heated by some of theexhaust gas supplied from the first channel 78 of the first heatingmechanism 77 a. The heat generated in the heat exchanger 18 is used forindirectly heating the reformer 22 by radiation or convection throughthe space 85.

Temperatures at the respective positions of the reformer 22 are detectedby the temperature sensors 130 a to 130 d attached to the reformer 22,and the temperature data of the detected temperatures t are inputted tothe control device 134. The control device 134 compares thepredetermined temperature T (e.g., 350° C.) as one of predeterminedreforming condition values with each of the detected temperatures t. Theopen/close valve 132 is opened or closed under control to achieve thestate where the detected temperature t=the predetermined temperature T.

Specifically, since the reformer 22 is indirectly heated by the heatexchanger 18, the temperature in the reformer 22 becomes uniform. Whenthe detected temperature t is lower than the predetermined temperature T(t<T), the open/close valve 132 is opened more widely in comparison withthe case where the detected temperature t is equal to the predeterminedtemperature T (t=T). Thus, the flow rate of the exhaust gas flowing fromthe exhaust gas channel 76 into the first channel 78 is increased, andthe reformer 22 is heated directly by the exhaust gas to the greaterextent. As a result, the temperature difference between the detectedtemperature t and the predetermined temperature T is reduced.Accordingly, the direct heating by the exhaust gas supplied from thefirst channel 78 and the indirect heating by the radiation heat or theconvection heat from the heat exchanger 18 are balanced.

Further, when the detected temperature t is higher than thepredetermined temperature (t>T), the open/close valve 132 is closednarrowly in comparison with the case where the detected temperature t isequal to the predetermined temperature T (t=T).

By the control as described above, even if the operating condition ofthe fuel cell system 10 is changed, it is possible to accuratelyregulate the temperature of the reformer 22 to the predeterminedtemperature T. By the control, the optimum reforming condition isachieved regardless of the operating condition of the fuel cell system10. Therefore, the size of the reformer 22 is reduced. Further, thetemperature of the reformer 22 becomes uniform at the predeterminedreforming condition value, i.e., at the temperature of 350° C., forexample. It is possible to determine the S/C ratio at 1.0. As a result,the S/C ratio can be lowered down to the coking limit.

Thus, the amount of supplied water is reduced significantly. Thecapacity of the water supply power source such as the water pump and theconsumption amount of water is reduced significantly. The metal memberon the anode 34 side is not oxidized significantly by water vapor. As aresult, improvement in the durability is achieved. Since it is notnecessary to supply the water excessively, power generation in the fuelcell stack 12 can be performed efficiently.

Moreover, since the amount of supplied water is reduced, the amount ofwater vapor in the exhaust gas is reduced. Stated otherwise, it ispossible to reduce the latent heat of the water vapor which is consumedwastefully in the exhaust gas, heat loss is reduced and the waste heatis collected easily. Thus, no unduly high performance is required forthe heat exchanger 18 and the evaporator 20, and the sizes of the heatexchanger 18 and the evaporator 20 can be reduced.

Further, simply by operating the single open/close valve 132, it ispossible to maintain the state of the reformer 22 to have predeterminedreforming condition values. With the simple structure and process, thereforming reaction is carried out desirably.

Further, since the exhaust gas discharged from the fuel cell stack 12 isused as the heat source for directly and/or indirectly heating thereformer 22, the recovery rate of the waste heat is improvedeconomically.

Steam reforming is performed in the reformer 22, and in particular, thetemperature around the inlet 102 tends to be decreased. Therefore, bylocally heating the inlet 102 of the reformer 22 using the hot exhaustgas, it is possible to limit the decrease in the temperature of thereformer 22.

Further, in the first embodiment, the flow direction of the exhaust gasalong the heat exchanger 18 is substantially in parallel with the flowdirection of the mixed fuel along the reformer 22. In the heat exchanger18, the temperature on the inlet side is considerably low in comparisonwith the temperature on the outlet side. A predetermined temperaturegradient is generated in the longitudinal direction of the heatexchanger 18 indicated by the arrow A (see FIG. 10). In the reformer 22,the temperature around the inlet 102 tends to be the lowest due to thestrong endothermic reaction at the inlet 102.

In the heat exchanger 18, the temperature at the inlet is the highest.In the reformer 22, the temperature at the inlet 102 is the lowest. Theinlet of the heat exchanger 18 and the inlet 102 of the reformer 22 areprovided adjacent to each other, and the hot exhaust gas is supplied tothe inlet 102. In the structure, through the temperature gradient of theheat exchanger 18 in the direction indicated by the arrow A, thereformer 22 is heated so that the temperature in the reformer 22 becomesuniform in the longitudinal direction. The decrease in the temperatureat the inlet 102 of the reformer 22 is suppressed. Thus, the temperaturein the reformer 22 is maintained at a certain level, and the stabilityof the temperature is achieved. Therefore, the coking limit can besubstantially uniform in the entire reformer 22. By decreasing themaximum temperature in the reformer 22, it is possible to lower the S/Cratio.

Further, the fluid unit 14 including the heat exchanger 18, theevaporator 20, and the reformer 22 are provided on one side of the fuelcell stack 12, and the fluid unit 14 is provided symmetrically withrespect to the central axis of the fuel cell stack 12. Therefore, thefluid unit 14 having the high temperature in the fuel cell system 10 isprovided locally within the same area. Heat radiation from the fluidunit 14 is reduced. Thus, it is possible to increase the heat recoveryrate. Further, since the fluid unit 14 is provided symmetrically withrespect to the central axis of the fuel cell stack 12, significant heatstress or heat distortion is not generated, and improvement in thedurability is achieved.

Further, since the heat exchanger 18 and the reformer 22 are providednear the fuel cell stack 12, the heat is transferred from the fuel cellstack 12 easily and reliably. Accordingly, it is possible to increasethe heat recovery rate.

FIG. 11 is a cross sectional view showing main components of a fluidunit 150 of a fuel cell system according to a second embodiment of thepresent invention. The constituent elements that are identical to thoseof the fuel cell system 10 according to the first embodiment are labeledwith the same reference numeral, and description thereof will beomitted.

A fluid unit 150 includes a heat exchanger 18, a reformer 22, and anevaporator 152. The fluid unit 150 is provided on one side of the fuelcell stack 12, symmetrically with respect to the central axis of thefuel cell stack 12. In the fluid unit 150, the evaporator 152 isprovided outside the reformer 22, and the heat exchanger 18 is providedoutside the evaporator 152.

In the second embodiment, the evaporator 152 and the reformer 22 areprovided inside the heat exchanger 18. In the structure, it is possibleto heat the reformer 22 by the radiation heat and the convection heatfrom the heat exchanger 18. Improvement in the heat insulationperformance of the evaporator 152 is achieved effectively. It ispossible to produce the water vapor easily. Further, the dimension ofthe fluid unit 150 in the direction indicated by the arrow A is reducedeffectively. Accordingly, reduction in the overall size of the fuel cellsystem is achieved easily.

INDUSTRIAL APPLICABILITY

According to the present invention, the reformer is directly heated bysome of the exhaust gas, and indirectly heated by the heat exchanger towhich the remaining exhaust gas is supplied. By the direct heatingand/or the indirect heating using the exhaust gas, the state of thereformer is maintained to have the predetermined reforming conditionvalues reliably, and it is possible to lower the S/C ratio down to thecoking limit. Thus, the amount of supplied water is reducedsignificantly, and the load of the water supply power source such as thewater pump is reduced efficiently.

Further, simply by operating the single valve mechanism, the state ofthe reformer is maintained to have predetermined reforming conditionvalues. With the simple structure and process, the reforming reaction iscarried out to the greater extent. Further, by utilizing the exhaust gasas the heat source for directly and/or indirectly heating the reformer,significant improvement in the recovery rate of the waste heat isachieved economically.

1. A fuel cell system comprising: a fuel cell stack formed by stacking aplurality of fuel cells, said fuel cells each including an electrolyteelectrode assembly and a separator stacked together, said electrolyteelectrode assembly including an anode, a cathode, and an electrolyteinterposed between said anode and said cathode; a heat exchanger forheating an oxygen-containing gas to be supplied to said fuel cell stack;a reformer for reforming a mixed fuel of a raw fuel chiefly containinghydrocarbon and water vapor to produce a reformed gas; a first heatingmechanism for supplying some of an exhaust gas discharged from said fuelcell stack after consumption in power generation reaction to saidreformer as a heat source for directly heating said reformer; a secondheating mechanism for supplying the remaining exhaust gas to said heatexchanger as a heat source for heating the oxygen-containing gas, andsupplying the heat generated in said heat exchanger to said reformer asa heat source for indirectly heating said reformer; a temperaturedetection unit for detecting the temperature of said reformer; a valvemechanism for adjusting the flow rate of the exhaust gas supplied tosaid first heating mechanism; and a valve control mechanism foradjusting said valve mechanism based on the temperature of said reformerdetected by said temperature detection unit to control the ratio betweenthe amount of heat supplied from said first heating mechanism to saidreformer and the amount of heat supplied from said second heatingmechanism to said reformer in order to maintain the state of saidreformer to have a predetermined reforming condition value.
 2. A fuelcell system according to claim 1, wherein said reformer and said heatexchanger are provided near said fuel cell stack, and said heatexchanger is provided outside said reformer to form an indirect heatingspace of said second heating mechanism between said heat exchanger andsaid reformer.
 3. A fuel cell system according to claim 1, wherein saidreformer has an inlet and an outlet; the mixed fuel flows into saidreformer through said inlet, and the reformed gas after reforming issupplied to said fuel cell stack through said outlet; and said inlet isprovided adjacent to an exhaust gas outlet of said first heatingmechanism.
 4. A fuel cell system according to claim 1, wherein saidreforming condition value comprises the temperature of said reformer,and the molar ratio of carbon and the water vapor to the raw fuel.
 5. Afuel cell system according to claim 1, further comprising an evaporatorfor evaporating water to produce the mixed fuel, wherein a fluid unitincluding said heat exchanger, said evaporator, and said reformer isprovided on one side of said fuel cell stack, and said fluid unit isprovided symmetrically with respect to the central axis of said fuelcell stack.
 6. A fuel cell system according to claim 1, wherein the flowdirection of the remaining exhaust gas along said heat exchanger issubstantially in parallel with the flow direction of the mixed fuelalong said reformer.
 7. A method of operating a fuel cell systemincluding a fuel cell stack formed by stacking a plurality of fuelcells, said fuel cells each including an electrolyte electrode assemblyand a separator stacked together, said electrolyte electrode assemblyincluding an anode, a cathode, and an electrolyte interposed betweensaid anode and said cathode; a heat exchanger for heating anoxygen-containing gas to be supplied to said fuel cell stack; and areformer for reforming a mixed fuel of a raw fuel chiefly containinghydrocarbon and water vapor to produce a reformed gas, the methodcomprising the steps of: supplying some of an exhaust gas dischargedfrom said fuel cell stack after consumption in power generation reactionto said reformer as a heat source for directly heating said reformer;supplying the remaining exhaust gas to said heat exchanger as a heatsource for heating the oxygen-containing gas, and supplying the heatgenerated in said heat exchanger to said reformer as a heat source forindirectly heating said reformer; detecting the temperature of saidreformer; and adjusting the flow rate of the exhaust gas to be suppliedto said reformer based on the detected temperature of said reformer,thereby maintaining the state of said reformer to have a predeterminedreforming condition value.
 8. An operating method according to claim 7,wherein said reforming condition value comprises the temperature of saidreformer, and the molar ratio of carbon and the water vapor to the rawfuel.
 9. An operating method according to claim 7, wherein the flowdirection of the remaining exhaust gas along said heat exchanger) issubstantially in parallel with the flow direction of the mixed fuelalong said reformer.